[WIP] VecCore development branch + base geometry classes

This branch is work in progress. Please refrain from pushing anything to it, as I am rebasing it on the devel branch to keep history clean.

This branch contains a proposal for the backend system of the VecCore library and base geometry classes such as Point3D, Vector3D, Quaternion, Matrix3x3, and Transform. These classes will be used to test Quaternions as an alternative representation for rotations in VecGeom. The classes are ready in principle, but there might be bugs left that need fixing. I am currently working on a set of tests and benchmarks.

Please ignore the VecCore/simd/ directory for now. It needs to be updated with template specializations of the classes in VecCore/math/ for testing internal vectorization later. Now it just contains dead code.

This branch does not contain any changes outside of the VecCore/ directory.